A fundamental question in developmental neurobiology is how the vertebrate brain acquires its characteristic morphology to facilitate neuronal connectivity and organization. MicroRNAs (miRNAs) have recently emerged as novel regulators of gene expression with potential widespread influence on development and disease. To investigate the roles of miRNAs, the Giraldez laboratory has generated mutant zebrafish embryos (MZdicer) that are devoid of miRNA function. MZdicer embryos display normal patterning but fail to undergo proper neural tube morphogenesis and brain ventricle formation. Reintroduction of an early expressed miRNA (processed miR- 430) rescues these defects, indicating that miR-430 regulation is critical for proper morphogenesis. This proposal aims to determine how miR-430 governs gene expression to shape cell movement and organization during brain morphogenesis. Experiments outlined in Aim #1 will provide a detailed characterization of the MZdicer neural phenotype by combining time-lapse confocal microscopy with single-cell labeling and genetic mosaic analyses. These experiments will elucidate the onset and nature of defects in cell movement and organization that underlie the MZdicer neural phenotype. Experiments outlined in Aim #2 will identify the miR-430 target genes whose regulation is critical for proper brain morphogenesis. Time-course microarray and bioinformatic analyses will isolate mRNAs upregulated in the absence of miRNA function. Further, the in vivo functional relevance of individual miR-430-target mRNA interactions will be assessed through a combination of target misexpression and protection techniques. Taken together, these experiments will provide mechansitic insights into how miRNA activity instructs cellular behaviors during vertebrate brain morphogenesis. PUBLIC HEALTH RELEVANCE: Genetic malformations of the cerebral cortex cause severe mental health impediments, including retardation and epilepsy. In addition, failure of brain closure (anencephaly) and spinal closure (spina bifida) occur with a high prevalence in humans with devastating effects to nervous system function. This study has the potential to increase our understanding of the genetic basis of brain and spinal cord malformations by elucidating the regulatory processes that instruct neural tube and brain morphology during vertebrate development.